Matthew,
After trying d.setManageIp='20.20.20.20' on my system, i started getting "TypeError: 'str' object is not callable", when try to use d.setManageIp('20.20.20.20') again and could not get the device to change. After deleting the device and readding, the d.setManageIp('20.20.20.20') started working again. Not for sure what the d.setManageIp='20.20.20.20' is doing, but it broke my ability to change that devices IP address until I recreated the device in zenoss.
There is also a good partial script for you on the wiki (http://wiki.zenoss.org/ZenDMD_Tip_-_Zet_manageIp_via_Host_Lookup_%28A_record%29).
--Rob